High power radiation emitting devices, such as semiconductor lasers, typically comprise a body of semiconductor material having opposed end faces in which an active layer is positioned between two cladding regions of opposite conductivity. Gain, which is necessary for these high power devices, results from a population inversion which occurs when applied current is increased. The end faces of the body form a resonant cavity such that radiation generated in the active layer is partially reflected back into the semiconductor body by an end face toward the opposing end face. When the current is sufficiently increased above some threshold value the increase in gain causes lasing action to occur. Lasers emit a narrow band of highly coherent radiation having a coherence length of approximately 2 centimeters (cm). Coherent radiation, or radiation having narrow line width, is undesirable in some applications, such as fiber optic gyroscopes, which require high power devices which emit radiation having low coherence. Other devices such as light emitting diodes (LED's) emit a broad band of radiation but operate at low power, insufficient for high power applications.
Superluminescent diodes (SLDs) provide a high power output of broad band low coherent radiation, that being radiation having a coherence length of less than about 200 micrometers (.mu.m) and typically about 50 .mu.m. An SLD typically has a structure similar to that of a laser, with lasing being prevented by antireflection coatings formed on the end faces. These coatings must reduce the reflectivity of the end faces to about 10.sup.-5 or less to prevent lasing in a high power SLD and further, this reflectivity must be reduced to about 10.sup.-6 to achieve low spectral modulation. Spectral modulation is the percentage ratio of the difference between the maximum and minimum power output divided by the sum of the maximum and minimum power output and low spectral modulation is 5% or less modulation. Unfortunately, low reflectivity of about 10.sup.-6 at the end faces is difficult to obtain consistently for a given output wavelength and even a slight temperature change which alters the output wavelength will change the reflectivity, thus making the manufacture of low spectral modulation SLDs extremely difficult.
Other SLD structures utilize a stripe interrupt geometry in which a metallized stripe is formed over a portion of an active region. This stripe extends from one end face towards but not up to the opposing end face. The non-metallized region is supposed to be highly absorbing to greatly reduce reflection from the facet near that region. However the high optical field is known to bleach this absorbing region, making it somewhat transparent to the light propagating through it. As a result, facet reflection is not sufficiently eliminated, and the device exhibits high spectral modulation or even becomes a laser at high power.
Due to the aforementioned problems, an SLD has been limited to a maximum output power of about 7 mw continuous wave (cw) and has had high spectral modulation.
In U.S. patent application Ser. No. 040,977 filed Apr. 20, 1987, now U.S. Pat. No. 4,821,277 in the names of Alphonse and Gilbert, there is disclosed an angled stripe SLD where a gain guiding stripe is tilted with respect to the normal to the cleaved facet resulting in low reflectance in the optical beam path. For best results the angle of the tilt with respect to the normal has been less than 5.degree.. The efficiency of this SLD is limited by its lack of lateral confinement at the active region for the optical beam path. While it is known that higher efficiency devices can be obtained by using index-guided structures, this would result in some reflected rays being trapped in the optical beam path. To reduce the trapping of reflected rays the stripe angle would have to be larger than 5.degree.. However, a stripe angle larger than this angle is not practical because the corresponding high refraction angle in air would make fiber coupling to the SLD difficult.